The present invention relates in general to a puffer type gas circuit breaker and more particularly an improvement of puffer type gas circuit breakers provided with a pair of preinsertion resistor contacts.
When a circuit breaker inserted in extrahigh voltage transmission lines is closed, a switching overvoltage or switching surge voltage is generated, causing damage to the insulation of the electrical equipment in the transmission system. In order to overcome this problem, the power circuit breakers used in extrahigh voltage transmission systems are in general provided with a preinsertion resistor contact unit (to be referred to as "resistor contact unit" in this specification) electrically inserted in parallel with a pair of main movable and fixed contacts and comprised of a pair of movable and fixed resistor contacts and a preinsertion resistor assembly. The resistor contact unit closes the circuit through the preinsertion resistor assembly prior to the closing of the main movable and fixed contacts so that the generation of a switching overvoltage is prevented.
However, even though the resistor contact unit has a sufficient closing capacity, it does not have opening or interrupting capacity. Therefore, in general, the resistor contact unit should be opened before the interrupter unit is opened, and the extinction of the arc is carried out only between the main movable and fixed contacts of the interrupter unit. Therefore, the resistor contact unit must always have a higher dielectric strength than that of the interrupter unit through all of their transient opening strokes. Otherwise, the arc is re-established between the movable and fixed resistor contacts for which there is provided no means for extinguishing the arc so that the interruption cannot be attained. Therefore, when the circuit breaker is opened, the resistor contacts must be separated and spaced apart by a distance sufficient to prevent the transient dielectric breakdown between the resistor contacts before the main movable and fixed contacts of the interrupter unit are opened. If the closing or opening of the resistor contact unit is carried out concurrently of the closing or opening of the interrupter unit, the resistor contacts cannot be spaced apart from each other by a sufficient distance. Especially in the puffer type gas circuit breakers inserted in the extrahigh voltage transmission lines of the type which can accomplish the interruption within two cycles after it has received the tripping signal, the opening time of the main movable and fixed contacts is extremely short and is of the order of 10 to 20 milliseconds.
Another requirement for the circuit breaker is to operate with trip free duty, which means "close-open" operation.
Considering this case, it is extremely difficult to separate the resistor contacts by a sufficient distance within such an extremely short time prior to the opening of the main movable and fixed contacts, if they are operated concurrently by the same tripping signal coming immediately after closing operation.